Reconsider single-gate devices in designs

From a system design point of view, the use of standard logical structures as component units will still be an important part of the entire system design. Microprocessors, DSPs, ASICs, and custom circuits now occupy many design positions traditionally occupied by standard logic functions. However, due to the cost and complexity of adopting more advanced technology solutions, designers continue to use standard logic in specific applications.

A major application of standard logic is in "connecting things together." Discrete logic ICs are often called "glue logic". They enable individual parts of the system to communicate more efficiently with other parts. Simple circuit functions such as buffers, decoders, and switches continue to increase in design popularity and performance. Examples that require glue logic include circuit isolation, input/output conversion, power supply conversion, and single bus line conversion.

Single-gate logic devices are a derivative family of larger multi-gate devices. Initially, Japan was the first to use single-gate logic devices to solve design problems in the consumer electronics industry. Because Japan has a large number of consumer electronic devices, Japanese designers created a basic structure to support the rapid design of appropriately sized gate arrays and application-specific standard products (ASSPs).

Single-gate devices are becoming increasingly popular as designers strive to reduce board size. This device enables designers to place a logic function appropriately where it is needed in the system with minimal practical requirements. In fact, single-gate devices are small enough that designers can easily add a logic function to upgrade the original design without modifying the main system. Single-gate devices also enable designers to reduce power consumption, noise and crosstalk.

Single gate device driving force

The impetus for the creation of single-gate devices comes when an existing gate array or ASSP design needs to integrate a 1-bit buffer, logic, or switch into a new design. There is often not enough room on the board to add additional multiple gate logic devices to maintain the same board size. The designer's only options are to redesign the entire chip or add ICs to the board layout to achieve the desired functionality.

Additionally, companies making portable devices are subject to increasing pressure to reduce board size. Before the use of single-gate devices, the package size of traditional logic circuits depended on how the circuit board was designed and laid out.

Product process platforms and circuit design density often limit package size. For example, multi-gate devices in SSOP or TSSSOP packages occupy 50 and 80 mm2 board area respectively. With a small single-gate device, designers can adapt the circuit to new system designs. This has pushed chipmakers to produce single-gate versions of industry-standard multi-gate devices, and has also forced chip vendors to deliver the best single performance in the smallest possible space while saving as much power as possible.

The final push for component suppliers to produce and bring single-door products to market is time-to-market pressure. As the name suggests, the single-door functionality is implemented in an ultra-small package. These solutions were originally provided in SOT-23 (SC59) 5-pin packages, and later in the current SOT 353 (SC88A/SC70) packages. The area of ​​SOT353 is only 4.2mm2, which is 3% of the area required by traditional 20-pin SOIC. This package is small enough to be installed directly "in the line" of the traces.

Because designers can install single-gate devices exactly where they are needed, the direct benefits are: smaller ground echo effects, smaller decoupling components required, and shorter signal lines. The single-gate design greatly reduces overall board space and crosstalk, provides a cleaner system signal, and eliminates signal "cleaning" components previously required. Workstations and other non-portable products can also use single-gate devices to reduce board space and reduce power consumption.

Single gate device application

Single door technology has a range of applications. In each instance, a 3V logic level serial input must be interfaced to a 5V board. In this example, a single-gate device 1GT50 is used, and a non-inverting interface circuit is shown in Figure 1.

1GT50 operates at 5V and has seamless interface with 3V logic levels. No resistors or other additional components are required. This device is nearly load-free (less than 10pF) and can provide up to 8mA drive with minimal noise and ground echo and small signal delay (approximately 4ns, depending on load).

In another example, a motor-driven phase-locked loop (PLL) requires a fast startup time with a long steady-state time constant. A single-door analog switch (1GT66 or 1G66) does the trick (Figure 2).

Many designers are familiar with this functionality in multi-gate devices. The designer determines two time constants using either gate. The fast start-up time is the first constant, with about 15% overshoot. The second time constant has maximum stability and minimum ripple. Analog switches require a resistor and two capacitors. When the switch is turned on, the selected time constant is: t=2π (C1+C2). The longer time constant is effective a few nanoseconds after the switch is turned on.

In the next application, the application switches on a low-power 3.3V device from a TTL level source. A 1G66 switch can be used here. Connect Vdd to the 3.3V power supply as a high-side analog switch (Figure 3).

The control pins of the 1G66 are overvoltage tolerant and can be driven by a 5V logic driver. This switch has only a 15Ω resistor and a voltage drop of 0.15V into a 10mA load. This feature turns on the local oscillator, RF stage, small audio output, etc. This inexpensive switch provides the interface between the 5V portion of the system and the high-end switches. No external resistors or capacitors are required.

The next example determines how to build a small, low-cost crystal resonant oscillator. The 1GU04 unbuffered inverter can be used as an oscillator for any fundamental mode crystal. A 10MΩ resistor is connected between the output and input, placing the inverter in Class A (Figure 4). The crystal manufacturer should determine the capacitor value. The maximum frequency of this oscillator reaches 25MHz. Designers can use harmonic crystals to achieve higher frequencies. If buffering is required, any VHC single or multi-gate buffer or inverter will suffice.

Use an op amp with selectable feedback resistors to provide constant input and output impedance. This is the best way to build a dual-gain audio amplifier (0 or +6dB gain). Use a single-gate analog switch to select the resistor and provide 1 or +6dB gain (Figure 5).

For many years Programmable Array Logic (PAL) has been used to perform multi-signal telegram complex logic operations. In wireless/cell phones, PAL consumes quite a bit of power. When designers require complex "combinational" logic, another problem arises, namely how to achieve the surrounding state.

Depending on the required conditions, an open-drain single-gate device can provide a good solution. Open-drain gates allow the outputs to be wire-ORed together, which not only has an OR function, but also has very low power, small area, and almost no delay in entering the signal path (Figure 6).

Here is an example of a complex function:

OUT=(A0×A1)+(A2×A3)+(A4+A5)

Use three open-drain single-gate devices (09, 01, 03) for wired OR output. For this functionality, using PAL would consume too much power and board space. Using multi-gate logic will require 4 devices and multiple 50mm2 area. The open-circuit device using a drain plate only occupies a board area of ​​13mm2. In Figure 6, the minimum power consumption is determined by the R value, and its signal propagation delay is less than 7ns.